Device and method for determining leaks of a respirator

ABSTRACT

A method, system and device are provided for the determination of leaks in a respirator. An inspiraton pressure p insp  is recorded during an inspiration time T insp  with a measuring device. The inspiration pressure p insp  is varied during consecutive breaths I. Leaks are determined as to leak volumes V Li  from the product of p inspi  and T inspi  in accordance with the equation
 
 V   Li   =A×p   inspi   B   ×T   inspi  
 
while the parameters A and B are determined in an analyzing unit by a regression model with p insp  as the regressor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofGerman Patent Application DE 10 2005 061 439.6 filed Dec. 22, 2005, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to respirators (also known as ventilators)generally and more particularly to a method, system and device for thedetermination of leaks in a device for supplying a patient withbreathing gas, which has at least one analyzing unit and one measuringdevice for determining the respiratory tract pressure.

BACKGROUND OF THE INVENTION

In almost all respirators, leaks can occur, which are eithersystem-related, as a result of a mask that has not been put on tightlyenough, or patient-related, for example, in the form of a fistula. Thisleak generally depends on the positive pressure respiration and on therespiration method used. A distinction is to be made here betweenpressure-controlled and volume-controlled respiration.

In volume-controlled respiration, the inspiration takes place aspredetermined by the stroke volume and by the time curve of the volumeflow, and the respiratory tract pressure is dependent upon the volumeflow and the stroke volume. The level of the respiratory tract pressureessentially depends on the lungs of the patient and also on thebreathing efforts of the patient. In the volume-controlled respiration,it is necessary to monitor the upper respiratory tract pressure in orderto prevent a lung of the patient from becoming damaged due to too highpressures. The rigid predetermination of the time curve of the volumeflow has a problematic effect, if the patient develops his/her ownactivity during the breathing and thus wants to determine the volumeflow himself/herself.

Pressure-controlled respiration takes place as predetermined by the timecurve of the respiratory tract pressure. In this case, so much breathinggas is supplied until a predetermined respiratory tract pressure isreached. In pressure-controlled respiration, the volume flow and thestroke volume are monitored. If the breathing gas for respirating apatient is provided via a fan as a pressure source, then the respirationusually takes place as predetermined by the pressure curve, since therespiratory tract pressure, but not the volume flow can be adjusted viaa regulation of the speed of the fan in a relatively simple way. Thepossibility of also predetermining the stroke volume here is desirable.

To compensate for a leak in pressure-controlled respiration, a so-calledcompensating gas flow is added, so that the respiratory tract pressurecan be maintained at the preselected pressure level.

In the volume-controlled respiration, the added compensating gas flowshould be included in the calculation of the stroke volume. Thecompensation for a leak by means of adding a compensating gas flow alsointerferes with the setting of the trigger thresholds, with which aninspiration stroke is triggered.

Since the system leak has different effects on the systemfunctionalities of the respirator, accurate knowledge thereof isimportant in order to be able to make a suitable compensation. In theprior-art respirators, usually a linear or square root dependence of theleak is based on the respiration pressure.

A square root dependence between a leak and the respiration pressure fora respirator with a fan emerges, for example, from U.S. Pat. No.6,659,101 B2.

The drawback in this case is that a characteristic curve limited to thesquare root function can only approximate the different leak formsapproximately.

SUMMARY OF THE INVENTION

The object of the present invention is thus to provide an improvedmethod for leak compensation and a device for carrying out the method.

According to the invention, a method is provided for the determinationof leaks in a device for supplying a patient with breathing gas. Thedevice has at least one analyzing unit and one measuring device fordetermining the respiratory tract pressure. The method includesrecording an inspiration pressure p_(insp) during an inspiration timeT_(insp) with the measuring device and varying the inspiration pressurep_(inspi) during consecutive breaths I. Next the method includesdetermining leak volumes V_(Li) from the product of p_(inspi) andT_(inspi) in accordance with the equationV _(Li) =A×p _(inspi) ^(B) ×T _(inspi)while the parameters A and B are determined in the analyzing unit by aregression model with p_(insp) as the regressor.

The method may also include adding an additive term ε_(i), which takesirregular breathing of the patient into consideration, to the equationfor V_(Li).

Further, the method may also include the step of additionally takinginto account the end expiratory pressure peep as another regressor andthus allowing variable expiratory pressures according to the equationV _(Li) =A(p _(inspi) ^(B) ·T _(inspi)+peep_(i) ^(B) ·T _(exspi))+ε_(i).

According to another aspect of the invention a system and device areprovided for the determination of leaks in a device for supplying apatient with breathing gas. The device includes a means for producing arespiratory pressure curve, a means for switching between an inspirationphase and an expiration phase as well as a measuring device forrecording an inspiration pressure p_(insp). A means is provided fordescribing leak volumes V_(Li) from the product of p_(inspi) andT_(inspi) in accordance with the equationV _(Li) A×p _(inspi) ^(B) ×T _(inspi)with the parameters A and B to be determined by a regressioncalculation. A means is provided for determining the leak flow {dot over(V)}_(L)(t) from the positive pressure respiration curve p(t) accordingto the equation{dot over (V)} _(L)(T)=A·p ^(B)(t)with the parameters A and B to be determined by a regressioncalculation.

According to the present invention, instead of a arbitrary parametricfunction with at least n=2 parameters, an exponential function with twofree parameters A and B is used to describe the dynamic pressuredependence of the leak flow. The leak flow amounts to{dot over (V)} _(L)(T)=A·p ^(B)(t)  (1)and the leak volume V_(L)V _(L) =∫{dot over (V)} _(L)(t)dt=A∫p ^(B)(t)dt  (2)The volume balance ΔV generally appears from the equationΔV=V _(L) +ε=A∫p ^(B)(t)dt+ε,  (3)whereby the summand ε denotes irregular breathing. ΔV is breath-resolvedor averaged over several consecutive breaths. In a completely passive,e.g., sedated, patient, ΔV and V_(L) are identical.

In the general case, random positive pressure respiration curves p(t)are assumed. The volume balance and the corresponding respiratory tractpressure curve are measured and are stored for each breath, as is shown,for example, in Table 1.

TABLE 1 Measurements in the general case with random pressure curve.Volume balance Pressure time curve ΔV₁ p₁(t) ΔV₂ p₂(t) ΔV₃ p₃(t) . . . .. .By means of the model statement according to equation 3ΔV=A∫p ^(B)(t)dt+ε,  (4)the free parameters A and B can be estimated by means of using suitableregression methods. The prerequisite for an estimate is that enoughbreaths (n≧2) are measured and that the leak volumes and pressurecurves, in particular the maximum inspiration pressure, are sufficientlydifferent.

The method can be simplified if each breath is split into N intervalsand an average respiratory tract pressure is determined at each of theseintervals. The simplification is exact, i.e., a systematic error doesnot arise if the respiratory tract pressure curve does not deviate fromthe respective average respiratory tract pressure at the intervals,i.e., if the pressure curve can be described as a step function.

If the simplified case of two intervals (inspiration and expiration) anda step-like pressure curve are taken into account, then p(t)=p_(insp)during the inspiration phase and p(t)=peep in the expiration phase.

According to Table 2, the volume balance is determined for each breathboth for the inspiration and the expiration.

TABLE 2 Measurements for the simplified method with constant pressurecurve in the inspiration phase and the expiration phase. Volume balanceInspiration interval Expiration interval ΔV₁ P_(insp1), T_(insp1) peep₁,T_(exsp1) ΔV₂ P_(insp2), T_(insp2) peep₂, T_(exsp2) ΔV₃ P_(insp3),T_(insp3) peep₃, T_(exsp3) . . . . . . . . .From the general model statement according to equation 4,ΔV=A·(p _(insp) ^(B) ·T _(insp)+peep^(B) ·T _(exsp))+ε  (5)is obtained after the simplification.

As in the general case, the free parameters A and B can be estimated bymeans of suitable regression methods. The regression is thentwo-dimensional with p_(insp) and peep as regressors. The regression issimpler, if the peep is constant, i.e., if it does not change over thebreaths taken into account.

The simplest case is present, if peep=0, since the parameters A and Bcan be estimated here with linear regression.

According to Table 3, the volume balance and pressure and duration ofthe inspiration phase are determined for each breath.

TABLE 3 Measurements for the simplest case with rectangular pressurecurve during the inspiration (p_(insp)) and peep = 0 in the expirationphase Volume balance Inspiration interval ΔV₁ P_(insp1), T_(insp1) ΔV₂P_(insp2), T_(insp2) ΔV₃ P_(insp3), T_(insp3) . . . . . .From the model statement according to equation 5, the modelΔV=A·p _(insp) ^(B) ·T _(insp)+ε  (6)is obtained under the simplified marginal conditions (peep=0).Taking the logarithm of the equation results inlog(ΔV−ε)=log(A·T _(insp))+B log(p _(insp))  (7)and thus the possibility of the simple linear regression for determiningthe parameters A and B.

With idealized, rectangular pressure curve, the respirator produces aconstant excess pressure P_(insp) at the respiratory tract opening ofthe patient during the inspiration phase with the duration T_(insp). Itis assumed that the applied excess pressure falls back to zero in asufficiently short time after the exchange in the expiration phase, sothat P_(exp)=0 applies. The excess pressure in the inspiration phaseP_(insp) is determinant for the leak occurring there with the leakvolume V_(L), which can be caused by an incorrectly fitted respiratormask, a poorly situated tube or an unintended discharge opening. Ani^(th) inspiration phase is described by the pressure P_(inspi), theinspiration time T_(inspi) and by the leak volume V_(Li).

It is assumed that the inspiration pressure P_(insp) can vary frombreath to breath. Other examples where P_(inspi) varies from breath tobreath are: volume controlled ventilation; and proportional assistventilation (resp. proportional pressure support). The case of aninspiration pressure that is constant over all breaths taken intoaccount is discussed separately. A measured value each is taken for thepressure P_(inspi) and the inspiration time T_(inspi) for each breath i.The leak volume V_(Li) cannot generally be measured, but can only beestimated. The volume balance ΔV_(i) is an estimated value for V_(Li)that is true to expectations and deviates from this only because ofphysiological irregular breathing ε_(i). The volume balance ΔV_(i) isidentical to the leak volume V_(Li) in a passive, e.g., sedated patient.

Irregular breathing ε interferes with a determination of the dependenceof the leak volume V_(L) on the inspiration pressure P_(insp). For thisreason, the data of the breaths, which are characterized by significantirregular breathing, are discarded. The irregular breathing ε cannot bemeasured directly. However, since it is correlated with the expiratoryvolume V_(exp), the latter can be used as an indicator of the irregularbreathing. The breaths, in which the expiratory volume deviates sharplyfrom the “mean value,” are discarded. A median-based criterion is usedin this case: The breaths, in which the current measured value V_(expi)deviates from the median of the past m measured values by more than acertain percentage (20%), are eliminated.

According to the simplified model, it is assumed that the dependence ofthe leak flow on the inspiration pressure P_(insp) can be described bymeans of the exponential function. As in equation 6, the result for theleak volume V_(Li) is:V _(Li) =A·p _(insp) _(i) ^(B) ·T _(insp) _(i)   (8)Since the leak volume V_(Li) is not available as a measured value, thevolume balance ΔV_(i), interfered with by irregular breathing ε_(i), isused as an alternativeΔV _(i) =A·p _(insp) _(i) ^(B) ·T _(insp) _(i+ε) _(i)  (9)The free parameters of the function A and B can be estimated, e.g., bynonlinear regression or more simply using the following method:The logarithm is taken of equation (9), and a linear equation isobtainedlog(ΔV _(i)−ε_(i))−log T _(insp)=log A+B log p _(insp)  (10)in which the irregular breathing ε_(i) is no longer additive but is inthe logarithm. By means of Taylor expansion and termination after thefirst term,log ΔV _(i)−log T _(inspi)=log A+B log p _(inspi)+ε_(i) /ΔV _(i)  (11)is obtained approximately.

The left side can be represented abbreviated by y_(i) and the termlog(p_(inspi)) by x_(i).

It is assumed that the noise term standardized to ΔV_(i), i.e., residualirregular breathing standardized to the volume balance,E_(i)=ε_(i)/ΔV_(i), is random and not dependent on p_(inspi) or ΔV_(i).Simplifying, with a=log A, the result isy _(i) =a+Bx _(i) +E _(i)  (12)To determine the constants a and B, only the point of intersection withthe y axis and the gradient of the fitted straight lines must bedetermined. Classical linear regression is suitable for this and yieldsthe following result:

$\begin{matrix}{{B = \frac{{\left\langle x \right\rangle\left\langle y \right\rangle} - \left\langle {xy} \right\rangle}{{\left\langle x \right\rangle\left\langle x \right\rangle} - \left\langle {xx} \right\rangle}}{a = {{\left\langle y \right\rangle - {B\left\langle x \right\rangle\mspace{31mu} A}} = {\mathbb{e}}^{{\langle y\rangle} - {B{\langle x\rangle}}}}}} & (13)\end{matrix}$whereby the brackets represent averaged measured values over severalbreaths.

With the coefficients A and B now known, the leak flow can be calculateddirectly from the positive pressure respiration{dot over (V)} _(L) =A·p ^(B)(t).  (14)Since the pressure is constant in both the inspiration phase and theexpiration phase for the special simplest case,

$\begin{matrix}{{p(t)} = \begin{matrix}\left\{ {p_{insp}\mspace{14mu}{for}\mspace{14mu}{inspiration}\mspace{14mu}{phase}} \right\} \\\left\{ {0\mspace{14mu}{for}\mspace{14mu}{expiration}\mspace{14mu}{phase}} \right\}\end{matrix}} & (15)\end{matrix}$applies, and the corresponding result for the leak flow is

$\begin{matrix}{{{\overset{.}{V}}_{L}(t)} = \begin{matrix}\left\{ {{A \cdot p_{insp}^{B}}\mspace{14mu}{for}\mspace{14mu}{inspiration}\mspace{14mu}{phase}} \right\} \\\left\{ {0\mspace{14mu}{for}\mspace{14mu}{expiration}\mspace{14mu}{phase}} \right\}\end{matrix}} & (16)\end{matrix}$If the inspiration pressure is constant over all the breaths taken intoaccount, this would be, e.g., the case in conventional respiration withBIPAP at PEEP=0, only two points (zero point and measured point) areavailable for the regression according to equation 9, through which afitted curve shall be drawn. Thus, only a linear statement is possible,i.e., the exponent B is arbitrarily The linear statement is given onlyfor the sake of completeness and is not a subject of the presentinvention.

An exemplary embodiment of the present invention is shown in the drawingand explained in detail below. The various features of novelty whichcharacterize the invention are pointed out with particularity in theclaims annexed to and forming a part of this disclosure. For a betterunderstanding of the invention, its operating advantages and specificobjects attained by its uses, reference is made to the accompanyingdrawings and descriptive matter in which preferred embodiments of theinvention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic view showing the construction of a respirator withfeatures for practicing the invention;

FIG. 2 is a graph showing an example for a rectangular positive pressurerespiration curve;

FIG. 3 is a graph showing a first measurement curve for a leak;

FIG. 4 is a graph showing a second measurement curve for a leak;

FIG. 5 is a graph showing a third measurement curve for a leak.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in particular, FIG. 1 shows a respirator 1 forsupplying a patient 2 with breathing gas via an inspiration line 3. Theexhaled breathing gas reaches an expiration outlet 6 via an expirationline 4 and an expiration valve 5. The expiration valve 5, which sets anexpiration pressure p_(peep) during the expiration of the patient, isactuated by a linear drive 7, which is connected to an analyzing unit 8of the respirator 1 via a signal line 13. The inspiration line 3 and theexpiration line 4 merge into a Y piece 9, from which a joint breathinggas line 10 for the inspiration and expiration leads to the patient. Apressure sensor 11 for measuring the breathing gas pressure p and a flowsensor 12 for measuring the breathing gas flow V are arranged in theinspiration line 3 and are connected to the analyzing unit 8 via signallines 13. The analyzing unit 8 contains a central control unit of therespirator 1 with a microprocessor, which is not shown in detail in FIG.1, for storing and analyzing the data supplied by the measuring systems11, 12 and for controlling the breathing phases.

To the analyzing unit 8 is connected a set point transducer 14 for theinspiration pressure p_(insp) and for the expiration pressure p_(peep),which receives the values selected by the user. The breathing gas flowto the patient 2 is dispensed via a control valve 15, which is connectedto a pressure gas source, which is not shown in detail in FIG. 1, andreceives predetermined values for the inspiration pressure p_(insp) andthe expiration pressure p_(peep) from a ramp generator 16 during theinspiration. The leak flow V_(L) is illustrated by arrows 26 on a leakybreathing mask 22.

FIG. 2 schematically illustrates the time curve of the breathing gaspressure p(t). An idealized state is assumed, in which both theinspiration pressure p_(insp) and the expiration pressure p_(peep) isconstant. Two breathing phases with the inspiration phases p_(insp1) andp_(insp2) and the inspiration time T_(insp) are illustrated.

FIG. 3 illustrates an example of experimentally determined leaks ΔV_(i),which can be approximated by a linear function 27. The freak valuesshown in the right-hand area of FIG. 3 were eliminated by means of amedian-based criterion and were not taken into consideration in thedetermination of the fitted straight lines.

FIG. 4 shows another example of measured leak values, whose averagedcurve can be described by means of a square root function 28.

FIG. 5 illustrates leak values, which can be approximated by means of aparabolic function 29.

All three examples can be equally described by means of the exponentialfunction given according to the present invention. In this way, agreater range of variation of the approximation, which includes bothlinear and square root as well as parabolic leak forms, is obtained.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

1. A method for the determination of leaks in a device for supplying apatient with breathing gas, the method comprising the steps of:providing an analyzing unit and a measuring device for determining therespiratory tract pressure; recording an inspiration pressure p_(insp)during an inspiration time T_(insp) with the measuring device; varyingthe inspiration pressure p_(insp) during consecutive breaths i;determining leak volumes V_(Li) from the product of p_(inspi) andT_(inspi) in accordance with the equationV _(Li) =A×p _(inspi) ^(B) ×T _(inspi) while the parameters A and B aredetermined in the analyzing unit by a regression model with p_(insp) asthe regressor.
 2. A method in accordance with claim 1, furthercomprising the step of adding an additive term ε_(i), which takesirregular breathing of the patient into consideration, to the equationfor V_(Li).
 3. A method in accordance with claim 1, further comprisingthe step of additionally taking into account the end expiratory pressurepeep as another regressor and thus allowing variable expiratorypressures according to the equationV _(Li) =A(p _(inspi) ^(B) ·T _(inspi)+peep_(i) ^(B) ·T _(exspi))+ε_(i).
 4. A device for the determination of leaks in a device forsupplying a patient with breathing gas, the device comprising: a meansfor producing a respiratory pressure curve; a means for switchingbetween an inspiration phase and an expiration phase; a measuring devicefor recording an inspiration pressure p_(insp); and a means fordetermining leak volumes V_(Li) from the product of p_(inspi) andT_(inspi) in accordance with the equationV _(Li) =A×p _(inspi) ^(B) ×T _(inspi) with the parameters A and B to bedetermined by a regression calculation, wherein said means fordetermining leak volumes includes determining a leak flow {dot over(V)}_(L)(t) from the positive pressure respiration curve p(t) accordingto the equation{dot over (V)} _(L)(T)=A·p ^(B)(t) with the parameters A and B to bedetermined by a regression calculation.
 5. A method for thedetermination of leaks in a device for supplying a patient withbreathing gas, the method comprising the steps of: providing ananalyzing unit and a measuring device for determining the respiratorytract pressure; recording an inspiration pressure p_(insp) during aninspiration T_(insp) with the measuring device; determining parameters Aand B with said regression model in said analyzing unit, whereinp_(insp) is the regressor; varying the inspiration pressure p_(insp)during consecutive breaths i; determining leak volumes V_(Li) from theproduct of p_(inspi) and T_(inspi) based on the equationV _(Li) =A×p _(inspi) ^(B) ×T _(inspi).
 6. A method in accordance withclaim 5, further comprising the step of additionally taking into accountthe end expiratory pressure peep as another regressor and thus allowingvariable expiratory pressures according to the equationV _(Li) =A(p _(inspi) ^(B) ·T _(inspi)+peep_(i) ^(B) ·T _(inspi))+ε_(i).7. A method in accordance with claim 6, further comprising: determiningan expiratory volume for each breathe to provide a plurality ofexpiratory volume values; calculating a mean value based on saidplurality of expiratory volume values; determining whether one or moremeasured expiratory volume values is above a predetermined deviationrange with respect to said mean value to define one or more irregularbreathing values, wherein said step of determining said leak volumesV_(Li) does not include said irregular breathing values.
 8. A method inaccordance with claim 3, further comprising: determining an expiratoryvolume for each breathe to provide a plurality of expiratory volumevalues; calculating a mean value based on said plurality of expiratoryvolume values; determining whether one or more measured expiratoryvolume values is above a predetermined deviation range with respect tosaid mean value to define one or more irregular breathing values,wherein said step of determining said leak volumes V_(Li) does notinclude said irregular breathing values.